Bipolar transistor and method for producing the same

ABSTRACT

A bipolar transistor comprising a subcollector layer, and a collector layer on the subcollector layer. The collector layer includes a plurality of doped layers. The plurality of doped layers includes a first doped layer that has a highest impurity concentration thereamong and is on a side of or in contact with the subcollector layer. Also, the first doped layer includes a portion that extends beyond at least one edge of the plurality of doped layers in a cross-sectional view.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a Continuation of U.S. application Ser. No.17/229,564 filed Apr. 13, 2021, which is a Continuation of U.S.application Ser. No. 16/920,324 filed Jul. 2, 2020, which is aContinuation of U.S. application Ser. No. 16/710,957 filed Dec. 11,2019, which is a Continuation of U.S. application Ser. No. 16/375,724filed Apr. 4, 2019, which is a Continuation of U.S. application Ser. No.15/957,702 filed Apr. 19, 2018, which is a Continuation application ofU.S. patent application Ser. No. 15/694,111 filed Sep. 1, 2017, whichclaims benefit of priority to Japanese Patent Application 2016-245282filed Dec. 19, 2016, the entire content of which is incorporated hereinby reference.

TECHNICAL FIELD

The present disclosure relates to a bipolar transistor and a method forproducing the same.

BACKGROUND

In modern mobile communication terminals, heterojunction bipolartransistors (HBT) are commonly used as a component of a radio-frequencyamplifier module. HBTs are generally expected to meet the performancerequirements of, for example, high efficiency, high gain, high breakdownvoltage (high ruggedness upon load mismatch), and high output power. Foruse in second-generation cellular phones, HBTs with high ruggedness uponload mismatch are still in strong demand, but recently there has alsobeen a need for HBTs with higher output power. For use in third- andfourth-generation cellular phones, not only HBTs with high power addedefficiency but also ones with higher efficiency, high gain, and highoutput power are in demand. These trends indicate a growing need forhigher output power HBTs in recent years.

Japanese Unexamined Patent Application Publication Nos. 2006-60221 and2008-130586 disclose high output power HBTs, mentioning their structure.These HBTs have a substrate and a stack of subcollector, collector,base, and emitter layers on the substrate. The subcollector andcollector layers serve as an n-type collector region, the base layer asa p-type base region, and the emitter layer as an n-type emitter region.The collector layer is a stack of multiple doped layers with gradeddonor-impurity concentrations, higher on the subcollector layer side andlower on the base layer side. The portion of the emitter region throughwhich the emitter current actually flows is referred to as an intrinsicemitter region. In the base and collector regions, too, the currentflows through the portions lying beneath the intrinsic emitter region.The structure formed by the intrinsic emitter region and the portions ofthe base and collector regions lying therebeneath is referred to as anintrinsic HBT.

The HBT illustrated in FIG. 1A of Japanese Unexamined Patent ApplicationPublication No. 2006-60221 has a collector layer that includes first,second, and third n-type doped layers, from the closest to asubcollector layer. The first n-type doped layer has an impurityconcentration of 7×10¹⁶ cm⁻³ or more and 10×10¹⁶ cm⁻³ or less and athickness of 200 nm or more and 400 nm or less. The second n-type dopedlayer has an impurity concentration of 4×10¹⁶ cm⁻³ or more and 7×10¹⁶cm⁻³ or less and a thickness of 200 nm or more and 400 nm or less. Thethird n-type doped layer has an impurity concentration of 0.5×10¹⁶ cm⁻³or more and 4×10¹⁶ cm⁻³ or less and a thickness of 100 nm or more and500 nm or less. The subcollector layer has an impurity concentration of4×10¹⁸ cm⁻³ and a thickness of 400 nm.

In the HBT illustrated in FIG. 1C of Japanese Unexamined PatentApplication Publication No. 2006-60221, the collector layer has first,second, third, and fourth n-type doped layers, from the closest to asubcollector layer. The first n-type doped layer has an impurityconcentration of 7×10¹⁶ cm⁻³ or more and 10×10¹⁶ cm⁻³ or less and athickness of 200 nm or more and 400 nm or less. The second n-type dopedlayer has an impurity concentration of 4×10¹⁶ cm⁻³ or more and 7×10¹⁶cm³ or less and a thickness of 200 nm or more and 400 nm or less. Thethird n-type doped layer has an impurity concentration of 0.5×10¹⁶ cm⁻³or more and 4×10¹⁶ cm⁻³ or less and a thickness of 100 nm or more and500 nm or less. The fourth n-type doped layer has an impurityconcentration of 0.84×10¹⁶ cm⁻³ or more and 4×10¹⁶ cm⁻³ or less and athickness of 100 nm or more and 500 nm or less. The subcollector layerhas an impurity concentration of 4×10¹⁸ cm⁻³ and a thickness of 400 nm.

In the HBT illustrated in FIG. 20 of Japanese Unexamined PatentApplication Publication No. 2008-130586, the collector layer has first,second, and third n-type doped layers, from the closest to asubcollector layer. The first n-type doped layer has an impurityconcentration of 5×10¹⁶ cm⁻³ and a thickness of 200 nm. The secondn-type doped layer has an impurity concentration of 1×10¹⁶ cm⁻³ and athickness of 200 nm. The third n-type doped layer has an impurityconcentration of 5×10¹⁵ cm⁻³ and a thickness of 600 nm. The subcollectorlayer presumably has an impurity concentration of 1×10¹⁸ cm⁻³, althoughthis is speculation based on a description in an Example in thedisclosure.

As can be seen, in these HBTs, the subcollector layer has a highimpurity concentration, at least 1×10¹⁸ cm⁻³. In the fabrication of anHBT, it is a common practice to dope the subcollector layer to thehighest technically possible impurity concentration at the moment tominimize the collector resistance Rc, between the ends of the collectorelectrodes and the center of the collector layer. The collector layer isusually doped to a relatively low concentration, 1/10 or less of that inthe subcollector layer, for two purposes. One is to prevent thebase-collector capacitance from being too great, which would affect theefficiency, gain, and other radio-frequency characteristics of the HBT,and the other to prevent the base-collector and collector-emitterbreakdown voltages from being too low, which would cause the HBT to bebroken when operated to full radio-frequency power, in which its outputvoltage amplitude peaks.

Increasing the output power of an HBT requires reducing both collectorresistance and base-collector capacitance of the HBT. With the knowntechnologies, however, it is difficult to increase the output power ofan HBT by reducing both collector resistance and base-collectorcapacitance of the HBT. The following describes the reason withreference to FIGS. 11 and 12. FIGS. 11 and 12 illustrate known HBTs eachhaving a subcollector layer 20, a collector layer 30, a base layer 40,an emitter layer 50, an intrinsic HBT 110, a capping layer 120, acontact layer 80, an emitter electrode 51, collector electrodes 21, andbase electrodes 41.

As illustrated in FIGS. 11 and 12, the collector layer of the known HBTshas a multilayer structure, and this structure helps give the HBTs thedesired base-collector, collector-emitter, and on-state breakdownvoltages. The impurity concentration and thickness (typically,concentration distribution) of each doped layer constituting thecollector layer 30 determine these breakdown voltages. The totalthickness of the doped layers constituting the collector layer 30,however, has been disregarded, and some known HBTs have a collectorlayer 30 thicker than necessary for the desired breakdown voltages. Thebase-collector capacitance Cbc of a known HBT is composed of a depletionlayer capacitance Cbcd, external capacitances Cbcex1, and externalcapacitances Cbcex2. The depletion layer capacitance Cbcd is formedbetween the base layer 40 and the collector layer 30, the externalcapacitances Cbcex1 between the base electrodes 41 and base layer 40 andthe collector electrodes 21, and the external capacitances Cbcex2between the base electrodes 41 and base layer 40 and the subcollectorlayer 20. The depletion layer capacitance Cbcd makes a relatively largecontribution, but the other two types of external capacitances, Cbcex1and Cbcex2, also make non-negligible contributions.

If the collector layer 30 has the smallest thickness necessary for thedesired breakdown voltages, the base electrodes 41 and base layer 40 areclose to the collector electrodes 21 and subcollector layer 20, as inFIG. 11. This means that the external capacitances Cbcex1, formedbetween the base electrodes 41 and base layer 40 and the collectorelectrodes 21, are large, and so are the external capacitances Cbcex2,formed between the base electrodes 41 and base layer 40 and thesubcollector layer 20. As a result, the output power, and therefore thegain and efficiency, of the HBT are low.

Making the collector layer 30 thicker than necessary for the desiredbreakdown voltages is a way to avoid such large external capacitancesCbcex1 and Cbcex2. This, however, causes the problem of a large accessresistance of the collector layer 30, a layer having a lower impurityconcentration (about 1/10) than the subcollector layer 20. For betterunderstanding of this problem, the following describes some majorresistance components that contribute to the collector resistance Rc ofa known HBT with reference to FIG. 12. The contact resistance betweenthe collector electrodes 21 and the subcollector layer and theresistance of the collector electrodes are not illustrated. In thedrawing, the resistance components contributing to the collectorresistance Rc are expressed in a lumped element circuit for brevity,although it would be technically more accurate to use a distributedelement circuit.

The collector resistance Rc is composed of external subcollectorresistances Rscex, internal subcollector resistances Rscin, and accessresistances Rscac and Rcac. The external subcollector resistances Rscexare the resistances the subcollector layer 20 has in the areas from theends of the collector electrodes 21 to the ends of the collector layer30. The inner subcollector resistances Rscin are the resistances thesubcollector layer 20 has in the areas beneath the collector layer 30.The access resistance Rscac is the resistance to current flowing fromthe subcollector layer 20 to the active region of the intrinsic HBT 110.The access resistance Rcac is the resistance to current flowing from thesubcollector layer 20 to the region of the smallest thickness necessaryfor the desired breakdown voltages. The sum of an external subcollectorresistance Rscex and an inner subcollector resistance Rscin, referred toas an (Rscex+Rscin) resistance, has a width equal to the thickness ofthe subcollector layer 20 (typically between about 0.5 μm and about 1.5μm) and a length equal to the horizontal distance between the end of acollector electrode 21 to the center of the intrinsic HBT 110 (typicallybetween about 2 μm and about 4 μm).

The access resistance Rscac has the same width as the intrinsic HBT 110(typically between about 2 μm and about 6 μm) and the same length as thesubcollector layer 20 (typically between about 0.5 μm and about 1.5 μm).The contribution of the access resistance Rscac is therefore negligiblecompared with that of the (Rscex+Rscin) resistances. The contribution ofthe access resistance Rcac, however, cannot be ignored, because theaccess resistance Rcac, although identical in width to the intrinsic HBT110 (typically between about 2 μm and about 6 μm), extends over a lengthof about 0.3 μm to about 0.7 μm in the collector layer 30, in which theimpurity concentration is 1/10 or less of that in the subcollector layer20. It should be understood that the length and width of a resistance asmentioned herein refer to the lengths of the resistance as measuredparallel and perpendicular, respectively, to the direction of the flowof current.

The collector resistance Rc is therefore given by (Rscex+Rscin)/2+Rcac.This means that if the collector layer 30 is thicker than necessary forthe desired breakdown voltages, the access resistance Rcac, theresistance to current flowing from the subcollector layer 20 to theactive region of the intrinsic HBT 110, is accordingly large, and so isthe overall collector resistance Rc. As a result, the on-stateresistance Ron of the HBT cannot be lower than a certain limit, cappingthe output power of the HBT. Because of this tradeoff between theexternal capacitances Cbcex1 and Cbcex2 and the collector resistance Rc,it is difficult to increase the output power of an HBT by reducing bothof them, as long as the known structure continues being used.

SUMMARY

Accordingly, it is an object of the present disclosure to increase theoutput power of a bipolar transistor by reducing both collectorresistance and base-collector capacitance of the transistor.

According to preferred embodiments of the present disclosure, a bipolartransistor includes (i) a subcollector layer having first and secondsurfaces on opposite sides and collector electrodes on the firstsurface, (ii) a base layer having third and fourth surfaces on oppositesides and base electrodes on the third surface, (iii) a collector layerhaving fifth and sixth surfaces on opposite sides, with the fifth andsixth surfaces in contact with the fourth and first surfaces,respectively, and including multiple doped layers with graded impurityconcentrations, higher on the sixth surface side and lower on the fifthsurface side, and (iv) an emitter layer on the third surface. Themultiple doped layers include a first doped layer that has the highestimpurity concentration thereamong and is in contact with the firstsurface. The first doped layer has a sheet resistance less than or equalto about 9 times that of the subcollector layer.

Other features, elements, characteristics and advantages of the presentdisclosure will become more apparent from the following detaileddescription of preferred embodiments of the present disclosure withreference to the attached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view of the structure of a bipolartransistor according to an embodiment of the present disclosure.

FIG. 2 graphically represents the distribution of impurityconcentrations in some layers of a bipolar transistor according to anembodiment of the present disclosure.

FIG. 3 illustrates the components of the collector resistance of abipolar transistor according to an embodiment of the present disclosure.

FIG. 4 graphically represents the relationship between ρs1 c/ρssc andρstot/ρssc of a bipolar transistor according to an embodiment of thepresent disclosure.

FIG. 5 is a cross-sectional view of the structure of a bipolartransistor according to an example of the present disclosure.

FIG. 6 is a cross-sectional diagram illustrating a method for thefabrication of a bipolar transistor according to an example of thepresent disclosure.

FIG. 7 is a cross-sectional diagram illustrating a method for thefabrication of a bipolar transistor according to an example of thepresent disclosure.

FIG. 8 is a cross-sectional diagram illustrating a method for thefabrication of a bipolar transistor according to an example of thepresent disclosure.

FIG. 9 is a cross-sectional diagram illustrating a method for thefabrication of a bipolar transistor according to an example of thepresent disclosure.

FIG. 10 is a cross-sectional view of the structure of a bipolartransistor according to an example of the present disclosure.

FIG. 11 is a cross-sectional view of the structure of a known bipolartransistor.

FIG. 12 is a cross-sectional view of the structure of a known bipolartransistor.

DETAILED DESCRIPTION

The following describes some embodiments of the present disclosure withreference to the drawings. Like elements are given like numeralsthroughout and described only once in the following.

FIG. 1 is a cross-sectional view of the structure of a bipolartransistor 100 according to an embodiment of the present disclosure. Thebipolar transistor 100 includes a subcollector layer 2, a collectorlayer 3, a base layer 4, and an emitter layer 5, each being a layer of acompound semiconductor. The subcollector layer 2 has two principalsurfaces, one of which is referred to as the first surface 201 and theother as the second surface 202. The second surface 202 is on the sideopposite the first surface 201. On the first surface 201, of thesubcollector layer 2, are collector electrodes 9. The subcollector layer2 is on a substrate 1, with the second surface 202 in contact with thesubstrate 1. The base layer 4 also has two principal surfaces, one ofwhich is referred to as the third surface 401 and the other as thefourth surface 402. The fourth surface 402 is on the side opposite thethird surface 401. On the third surface 401 are the emitter layer 5 andbase electrodes 10. The collector layer 3 also has two principalsurfaces, one of which is referred to as the fifth surface 301 and theother as the sixth surface 302. The sixth surface 302 is on the sideopposite the fifth surface 301, with the fifth surface 301 in contactwith the fourth surface 402 and the sixth surface 302 in contact withthe first surface 201. The collector layer 3 includes multiple dopedlayers 31, 32, 33, and with graded impurity concentrations, higher onthe sixth surface 302 side and lower on the fifth surface 301 side. Thedoped layers 31, 32, 33, and 34 are stacked in this order on thesubcollector layer 2 and referred to as the first, second, third, andfourth doped layers, respectively. These doped layers vary in impurityconcentration but are of the same material. On the emitter layer 5 is astack of a capping layer 12, a contact layer 8, and an emitter electrode11.

The bipolar transistor 100 is, for example, a hetero-bipolar transistor,in which the emitter layer 5 and the base layer 4 form a heterojunctionand the emitter layer 5 has a band gap greater than that of the baselayer 4. The heterojunction reduces the base resistance, improving theradio-frequency characteristics of the bipolar transistor 100.Furthermore, the compound semiconductors give the bipolar transistor 100high electron mobility. The region 101 is referred to as an intrinsicHBT.

FIG. 2 graphically represents the distribution of impurityconcentrations in some layers of the bipolar transistor 100. In FIG. 2,the horizontal axis is for depth in the direction from the third surface401, of the base layer 4, to the first surface 201, of the subcollectorlayer 2, and the vertical axis for the impurity concentration in eachlayer. The segments 200, 310, 320, 330, 340, and 400 correspond to thedistributions of impurity concentrations in the subcollector layer 2,first doped layer 31, second doped layer 32, third doped layer 33,fourth doped layer 34, and base layer 4, respectively. The doped layers31, 32, 33, and 34 have a first type of conductivity, and the base layer4 has a second type of conductivity, opposite the first. For example,when the first type of conductivity is n-type, the second is p-type.When the first type of conductivity is p-type, for example, the secondis n-type. As illustrated, the first doped layer 31 has the highestimpurity concentration among the multiple doped layers 31, 32, 33, and34. The impurity concentration in the second doped layer 32 is thesecond highest, and that in the third doped layer 33 is the thirdhighest. In the fourth doped layer 34, the impurity concentration is thelowest. The impurity concentration in the first doped layer 31 and thatin the subcollector layer 2 may be the same or different.

Preferably, the impurity concentration in the subcollector layer 2 isabout 1×10¹⁸ cm⁻³ or more. Doping the subcollector layer 2 to a highconcentration will reduce the collector resistance Rc of the bipolartransistor 100, increasing the output power of the bipolar transistor100.

Desirably, each of the second, third, and fourth doped layers 32, 33,and 34 has an impurity concentration at least about an order ofmagnitude smaller than that in the first doped layer 31. This improvesthe base-collector and collector-emitter breakdown voltages, ensuringthe bipolar transistor 100 is not broken even when operated to fulloutput power.

For the second and third doped layers 32 and 33, it is preferred thatthe impurity concentration be about 1×10¹⁶ cm⁻³ or more and about 7×10¹⁶cm⁻³ or less. For the fourth doped layer 34, it is preferred that theimpurity concentration be about 3×10¹⁵ cm⁻³ or less. Under suchconditions, increasing the collector voltage Vc makes the base-collectordepletion layer rapidly expand within the fourth doped layer 34, owingto the impurity concentration in the fourth doped layer 34 much lowerthan those in the second and third doped layers 32 and 33. At a certainlow voltage Vca within the saturation region of the bipolar transistor100, the base-collector depletion layer reaches the boundary between thethird and fourth doped layers 33 and 34. In the third doped layer 33,however, the expansion of the base-collector depletion layer atcollector voltages Vc higher than or equal to Vca is limited because ofthe impurity concentration higher than that in the fourth doped layer34. This means that at collector voltages Vc higher than or equal toVca, the collector-voltage dependence of the base-collector capacitanceCbc is limited, and the linearity of the base-collector capacitance Cbcis improved. In this way, this adjustment of impurity concentrationsmakes the bipolar transistor 100 suitable for the application of RF(radio-frequency) signals for those telecommunication standards thatrequire high linearity, such as WCDMA® (Wideband Code Division MultipleAccess) and LTE (Long Term Evolution).

Preferably, the impurity concentration in the second doped layer 32 ishigher than that in the third doped layer 33. This makes the accessresistance R2 cac in the second doped layer lower than it is when thesecond and third doped layers 32 and 33 have the same impurityconcentration. The decrease in the access resistance R2 cac leads to adecrease in the overall collector resistance Rc of the bipolartransistor 100. The on-state resistance of the bipolar transistor 100 isreduced, and, as a result, the output power of the bipolar transistor100 is increased. Doping the second doped layer 32 to a highconcentration, furthermore, will reduce the loss of on-state breakdownvoltage where a large amount of current flows through the bipolartransistor 100, ensuring that even if the load changes when the bipolartransistor 100 is operated to full output power, the collector breakdownvoltage upon load mismatch, determined by the on-state breakdownvoltage, decreases only to a limited extent.

In each of the doped layers 31, 32, 33, and 34, the impurityconcentration does not need to be uniform and may have a gradient. Thenumber of doped layers constituting the collector layer 3 does not needto be four and can be two, three, five, or more.

FIG. 3 illustrates the components of the collector resistance Rc of thebipolar transistor 100. The first doped layer 31 has an impurityconcentration and thickness similar to those of the subcollector layer 2and extends beyond the boundary that separates the intrinsic HBT 101from the outside. The first doped layer 31 therefore behaves as if it isa low-resistance current path, which is not present in an HBT in theknown structure. Each of the resistance components the first doped layer31 has is denoted by R1 cin. Since each resistance component R1 cin isconnected in parallel to an internal subcollector resistance componentRscin, which lies in the subcollector layer 2, the collector resistanceRc is equal to (Rscex+Rscin/R1 cin)/2, where Rscin/R1 cin=(Rscin×R1cin)/(Rscin+R1 cin). The contribution of Rscac is again negligiblecompared with that of Rscex+Rscin/R1 cin, and so is that of R1 cac, forthe reason described above. R2 cac, in FIG. 3, is inside the thicknessrequired to achieve the desired breakdown voltages. It does not need tobe discussed and can be ignored.

The term “similar” as used in expressions like “A is similar to B”herein means that values A and B expressed as powers of ten have thesame exponent value. For example, when the value B is about 1/10 or moreand about 9 times or less the value A, the values A and B can be deemedas similar.

Rscin/R1 cin<Rscin, and Rscac and R1 cac are negligible. The collectorresistance Rc in this embodiment, (Rscex+Rscin/R1 cin)/2, is thereforealways smaller than that in the known structure, (Rscex+Rscin)/2+Rcac.Since the first doped layer 31 has an impurity concentration andthickness similar to those of the subcollector layer 2, the bipolartransistor 100 has a structure in which the subcollector layer 2 and thefirst doped layer 31 are connected in parallel. This parallel connectionreduces the collector resistance Rc. In an HBT in the known structure,the doped layer corresponding to the first doped layer 31 has a lowimpurity concentration compared with the subcollector layer, and,therefore, the resistance components in the subcollector layerpredominantly determine the collector resistance. Hence it is difficultto reduce the collector resistance of an HBT in the known structure.

The sheet resistance of the subcollector layer 2 is denoted by ρssc,that of the first doped layer 31 by ρs1 c, and the total sheetresistance of the subcollector layer 2 and the first doped layer 31,connected in parallel, by ρstot. FIG. 4 graphically represents therelationship between ρs1 c/ρssc and ρstot/ρssc of a bipolar transistor100 according to this embodiment. As the graph indicates, ρstot/ρssc isasymptotic to 1 at sufficiently high ρs1 c/ρssc ratios, about 0.9 at aρs1 c/ρssc of about 9, and sharply drops at lower ρs1 c/ρssc ratios.This means that bringing down ρs1 c/ρssc to about 9 or less is effectivein reducing the collector resistance Rc. At sufficiently low ρs1 c/ρsscratios, ρstot/ρssc is asymptotic to 0. Given the modest decrease, fromabout 0.1 to about 0, in ρstot/ρssc within the range of ρs1 c/ρssc lessthan about 1/10, ρs1 c/ρssc values lower than about 0.1 have littleeffect in reducing the collector resistance Rc. Since such low ρs1c/ρssc ratios are also technically difficult to achieve, it is preferredthat ρs1 c/ρssc be about 1/10 or more. Overall, it is preferred that ρs1c/ρssc be about 1/10 or more and about 9 or less. That is, it ispreferred that the first doped layer 31 have a sheet resistance about1/10 or more and about 9 times or less that of the subcollector layer 2.This means that if the first doped layer 31 and the subcollector layer 2have the same impurity concentration, it is preferred that the firstdoped layer 31 have a thickness about 1/9 or more and about 10 times orless that of the subcollector layer 2.

At ρs1 c/ρssc ratios of about 3 or less, ρstot/ρssc changes greatly witha change in ρs1 c/ρssc and is about 0.75 or less. This means thatbringing down ρs1 c/ρssc to about 3 or less reduces the collectorresistance Rc significantly. ρs1 c/ρssc ratios lower than about ⅓,however, have little effect in reducing the collector resistance Rc. Inthis range, the decrease in ρstot/ρssc is modest, from about 0.25 toabout 0. Overall, it is preferred that ρs1 c/ρssc be about ⅓ or more andabout 3 or less. That is, it is preferred that the first doped layer 31have a sheet resistance about ⅓ or more and about 3 times or less thatof the subcollector layer 2. This means that if the first doped layer 31and the subcollector layer 2 have the same impurity concentration, it ispreferred that the first doped layer 31 have a thickness about ⅓ or moreand about 3 times or less that of the subcollector layer 2.

Furthermore, forming the first doped layer 31 to a thickness similar tothe subcollector layer 2 and in contact with the first surface 201, ofthe subcollector layer 2, as in FIG. 3 places the base electrodes 10 andbase layer 4 farther away from the collector electrodes 9, reducing theexternal capacitances Cbcex1, and also from the subcollector layer 2,reducing the external capacitances Cbcex2. These reductions in theexternal capacitances Cbcex1 and Cbcex2 lead to a decrease in theoverall base-collector capacitance Cbc of the bipolar transistor 100.The bipolar transistor 100 combines a low collector resistance Rc with alow base-collector capacitance Cbc in this way, and offers high outputpower, high gain, and high efficiency.

Examples

FIG. 5 is a cross-sectional view of the structure of a bipolartransistor 100 according to this example. As illustrated in the drawing,the bipolar transistor 100 has a semi-insulating GaAs substrate 1 and astack of an n-type GaAs subcollector layer 2, an n-type GaAs collectorlayer 3, a p-type GaAs base layer 4, and an n-type In_(x)Ga_(1-x)Pemitter layer 5 on the substrate 1. The n-type GaAs subcollector layer 2has an Si concentration of about 2×10¹⁸ cm⁻³ or more and about 6×10¹⁸cm⁻³ or less and a thickness of about 0.3 μm or more and about 1.0 μm orless. The n-type GaAs collector layer 3 has a thickness of about 900 nmor more and about 1500 nm or less. The p-type GaAs base layer 4 has a Cconcentration of about 2×10¹⁹ cm⁻³ or more and about 5×10¹⁹ cm⁻³ or lessand a thickness of about 50 nm or more and about 150 nm or less. Then-type In_(x)Ga_(1-x)P emitter layer 5 has a Si concentration of about2×10¹⁷ cm⁻³ or more and about 5×10¹⁷ cm⁻³ or less and a thickness ofabout 30 nm or more and about 50 nm or less. The proportion of In, x, isabout 0.5.

On the n-type In_(x)Ga_(1-x)P emitter layer 5 is a stack of an n-typeGaAs layer 6, an n-type In_(x)Ga_(1-x)As grading layer 7, and an n-typeIn_(x)Ga_(1-x)As contact layer 8. The n-type GaAs layer 6 has a Siconcentration of about 2×10¹⁸ cm⁻³ or more and about 4×10¹⁸ cm⁻³ or lessand a thickness of about 50 nm or more and about 150 nm or less. Then-type In_(x)Ga_(1-x)As grading layer 7 has an Si concentration of about1×10¹⁹ cm⁻³ or more and about 3×10¹⁹ cm⁻³ or less and a thickness ofabout 30 nm or more and about 70 nm or less. The proportion of In, x, isabout 0 on the side closer to the p-type GaAs base layer 4 and about 0.5on the side farther from the p-type GaAs base layer 4. The n-typeIn_(x)Ga_(1-x)As contact layer 8 has a Si concentration of about 1×10¹⁹cm⁻³ or more and about 3×10¹⁹ cm⁻³ or less and a thickness of about 30nm or more and about 70 nm or less. The proportion of In, x, is about0.5.

The impurity concentration and thickness of the n-type In_(x)Ga_(1-x)Pemitter layer 5 are selected so that this layer is depleted of freeelectrons outside the area beneath the n-type In_(x)Ga_(1-x)As contactlayer 8, n-type In_(x)Ga_(1-x)As grading layer 7, and n-type GaAs layer6. Actually, therefore, current flows only through the intrinsic emitterregion 51, the portion of the n-type In_(x)Ga_(1-x)P emitter layer 5beneath the mesa of the n-type In_(x)Ga_(1-x)As contact layer 8, n-typeIn_(x)Ga_(1-x)As grading layer 7, and n-type GaAs layer 6. It should benoted that FIGS. 1 and 3 illustrate the emitter layer 5 as ifsubstantially all of it is the intrinsic emitter region. The depletedregion of the emitter layer 5 is not illustrated.

The first, second, third, and fourth doped layers 31, 32, 33, and 34,constituting the collector layer 3, are formed integrally into a mesa asa whole. No additional operation is therefore needed to form the firstdoped layer 31. The first, second, third, and fourth doped layers 31,32, 33, and 34 are n-type GaAs layers with different impurityconcentrations.

Preferably, the first doped layer 31 has an impurity concentration andthickness similar to those of the subcollector layer 2. This reduces thecollector resistance Rc in accordance with Rc=(Rscex+Rscin/R1 cin)/2.For example, it is preferred that the first doped layer 31 have animpurity concentration of about 1×10¹⁸ cm⁻³ or more and about 5×10¹⁸cm⁻³ or less, such as about 3×10¹⁸ cm⁻³, and a thickness of about 200 nmor more and about 900 nm or less, such as about 500 nm.

As for the second, third, and fourth doped layers 32, 33, and 34, it ispreferred that each have an impurity concentration at least about anorder of magnitude smaller than that in the subcollector layer 2. Thesecond doped layer 32 preferably has an impurity concentration of about3×10¹⁶ cm⁻³ or more and about 7×10¹⁶ cm⁻³ or less, such as about 5×10¹⁶cm⁻³, and a thickness of about 100 nm or more and about 300 nm or less,such as about 200 nm. The third doped layer 33 preferably has animpurity concentration of about 1×10¹⁶ cm⁻³ or more and about 4×10¹⁶cm⁻³ or less, such as about 1.5×10¹⁶ cm⁻³, and a thickness of about 100nm or more and about 300 nm or less, such as about 220 nm. The fourthdoped layer 34 preferably has an impurity concentration of about 3×10¹⁵cm⁻³ or less, such as about 3×10¹⁵ cm⁻³, and a thickness of about 300 nmor more and about 500 nm or less, such as about 400 nm.

On the surface of the n-type In_(x)Ga_(1-x)As contact layer 8 is anemitter electrode 11. The emitter electrode 11 is, for example, a Ti(about 50 nm thick)/Pt (about 50 nm thick)/Au (about 200 nm thick)electrode. On the surface of the p-type GaAs base layer 4 is a pair ofbase electrodes 11 facing each other with the intrinsic emitter region51 therebetween. The base electrodes 10 are, for example, Ti (about 50nm thick)/Pt (about 50 nm thick)/Au (about 200 nm thick) electrodes. Onthe surface of the subcollector layer 2 is a pair of collectorelectrodes 9 facing each other with the collector layer 3 therebetween.The collector electrodes 9 are, for example, AuGe (about 60 nm thick)/Ni(about 10 nm thick)/Au (about 200 nm thick)/Mo (about 10 nm thick)/Au(about 1 μm thick) electrodes.

The following describes a method for the fabrication of a bipolartransistor 100 with reference to FIGS. 6 to 9.

First, as illustrated in FIG. 6, an n-type GaAs subcollector layer 2 isformed on the surface of a GaAs substrate 1. Then first, second, third,and fourth doped layers 31, 32, 33, and 34 are sequentially formed onthe n-type GaAs subcollector layer 2 by the same process, giving ann-type GaAs collector layer 3. This way of forming the first doped layer31 and the other doped layers 32, 33, and 34, sequentially and by thesame process, allows the manufacturer to use an existing fabricationmethod as it is, requiring no additional operation to form the firstdoped layer 31. Then a p-type GaAs base layer is formed on the fourthdoped layer 34, an n-type In_(x)Ga_(1-x)P emitter layer 5 on the p-typeGaAs base layer 4, an n-type GaAs layer 6 on the n-type In_(x)Ga_(1-x)Pemitter layer 5, an n-type In_(x)Ga_(1-x)As grading layer 7 on then-type GaAs layer 6, and an n-type In_(x)Ga_(1-x)As contact layer 8 onthe n-type In_(x)Ga_(1-x)As grading layer 7. The individual layers 2 to8 of the bipolar transistor 100 are formed by an epitaxial process, suchas metal-organic chemical vapor deposition. The dopant for n-typesemiconductor layers can be, for example, Si, and that for p-typesemiconductors can be, for example, C. The n-type In_(x)Ga_(1-x)Ascontact layer 8 may be doped with Se or Te so that it has a highimpurity concentration.

Then, as illustrated in FIG. 7, an emitter electrode 11 is formed on thesurface of the n-type In_(x)Ga_(1-x)As contact layer 8. The n-typeIn_(x)Ga_(1-x)As contact layer 8, n-type In_(x)Ga_(1-x)As grading layer7, and n-type GaAs layer 6 are then etched, with the etch mask being aphotoresist mask (not illustrated), to leave the portion above theintrinsic emitter region 51 and eliminate the unnecessary rest. As aresult, a mesa of the n-type In_(x)Ga_(1-x)As contact layer 8, n-typeIn_(x)Ga_(1-x)As grading layer 7, and n-type GaAs layer 6 is determined.

Then, as illustrated in FIG. 8, the n-type In_(x)Ga_(1-x)P emitter layer5, p-type GaAs base layer 4, and collector layer 3 are etched, with theetch mask being a photoresist mask (not illustrated), to form a mesa andeliminate the unnecessary portion. The first, second, third, and fourthdoped layers 31, 32, 33, and 34 are sequentially etched by the sameprocess, and the resulting doped layers 31, 32, 33, and 34 havesubstantially the same two-dimensional shape when viewed in thedirection in which the doped layers 31, 32, 33, and 34 are stacked. Thisway of forming the first doped layer 31, into substantially the sametwo-dimensional shape as the other doped layers 32, 33, and 34, allowsthe manufacturer to use an existing fabrication method as it is,requiring no additional operation to form the first doped layer 31. Then-type In_(x)Ga_(1-x)P emitter layer 5 is then worked to expose theareas of the p-type GaAs base layer 4 in which base electrodes 10 are tobe formed. After that, base electrodes 10 are formed in contact with thep-type GaAs base layer 4 and alloyed to create ohmic contacts.

Then, as illustrated in FIG. 9, collector electrodes 9 are formed incontact with the subcollector layer 2 and alloyed to create ohmiccontacts. Lastly, the entire surface of the bipolar transistor 100 iscovered with a passivation coating 14, such as a SiN film.

The combination of the materials for the emitter layer and base layer 4does not need to be InGaP (emitter)/GaAs (base). For the emitter layer 5and base layer 4, other heterojunction-forming combinations of materialscan be also used including AlGaAs (emitter)/GaAs (base), InP(emitter)/InGaAs (base), InGaP (emitter)/InGaAs (base), InGaP(emitter)/GaAsSb (base), InGaP (emitter)/AlGaAs (base), InGaP(emitter)/InGaAsN (base), Si (emitter)/SiGe (base), and AlGaN(emitter)/GaN (base).

The first doped layer 31 may have, as illustrated in FIG. 10, atwo-dimensional shape that extends beyond the edges of thetwo-dimensional shape of the other doped layers 32, 33, and 34 whenviewed in the direction in which the doped layers 31, 32, 33, and 34 arestacked. This increases the area of contact between the subcollectorlayer 2 and the first doped layer 31, providing a further reduction incollector resistance Rc.

While preferred embodiments of the disclosure have been described above,it is to be understood that variations and modifications will beapparent to those skilled in the art without departing from the scopeand spirit of the disclosure. The scope of the disclosure, therefore, isto be determined solely by the following claims.

What is claimed is:
 1. A bipolar transistor comprising: a subcollectorlayer; a collector layer above the subcollector layer, the collectorlayer including a plurality of parts, the plurality of parts havingimpurity concentrations different from each other; and a collectorelectrode on the subcollector layer; wherein the collector layerincludes a portion that extends beyond at least one edge of theplurality of parts in a cross-sectional view, the portion is separatedfrom the collector electrode, the plurality of parts has graded impurityconcentrations, and the plurality of parts includes a first part, andthe impurity concentration in the first part is higher than an impurityconcentration in the subcollector layer.
 2. The bipolar transistoraccording to claim 1, wherein the impurity concentration in the firstpart is a highest impurity concentration thereamong and is on a side ofor in contact with the subcollector layer.
 3. The bipolar transistoraccording to claim 1, wherein the impurity concentration in the firstpart is higher than an impurity concentration in at least one of theplurality of parts and is on a side of or in contact with thesubcollector layer.
 4. The bipolar transistor according to claim 1,wherein the impurity concentrations of the plurality of parts are higheron a side of the subcollector layer and lower on a side opposite to thesubcollector layer.
 5. The bipolar transistor according to claim 2,wherein the first part comprises the portion.
 6. The bipolar transistoraccording to claim 3, wherein the first part comprises the portion. 7.The bipolar transistor according to claim 1, wherein the subcollectorlayer contacts the collector electrode.